Liquid withdrawing tray for purification column, purification column, and method for purification

ABSTRACT

In the purification of an easily polymerizable compound such as (meth)acrylic acid by the use of a purification column adapted to withdraw a liquid from the middle portion of the column, a novel liquid withdrawing tray adapted to prevent the compound from polymerization effectively and permit the purification to be stably performed for a long time, a purification column using this tray, and a method for purifications using this purification column are provided. The liquid withdrawing tray for the purification column is composed of a liquid receiving plate and a liquid withdrawing tube for withdrawing the liquid from the liquid receiving plate.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a liquid withdrawing tray for apurification column, a purification column, and a method forpurification. More particularly, it relates to a tray for withdrawing aliquid from a middle portion or middle level of a purification column, apurification column furnished with the tray, and a method forpurification an easily polymerizable compound such as (meth)acrylic acidby the use of the purification column.

[0003] 2. Description of the Prior Art

[0004] As a method for production of acrylic acid and esters thereof,the method by oxidizing propylene and/or acrolein with a molecularoxygen-containing gas in vapor phase, recovering a reaction product gasthus obtained with a solvent, and subjecting a reactionproduct-containing solution thus obtained purification in a purificationcolumn to obtain purified acrylic acid or esters thereof(JP-A-9-227445).

[0005] In the purification column, such operations as withdrawing aswithdrawing reflux liquid in the column from the middle portion,generally the middle level, of the column for the purpose of withdrawingan intermediate product, and cooling or heating the reflux liquid areroutine. Then, for the purpose of withdrawing the liquid from the middleportion of the purification column, the chimney tray which is composedof a vapor conduit (gas riser), an umbrella covering the conduit, and aliquid withdrawing tube has been popularly used (refer, for example, toHenry Z. Kister, “Distillation Operation,” pp. 102-116 [published in1990 by McGraw Hill, Inc.]). The chimney tray which is concurrentlyprovided with a vapor conduit and a liquid withdrawing tube is alsoavailable for the purpose of the withdrawal of interest.

[0006] In purification of such an easily polymerizable compound as(meth)acrylic acid, the measure taken to polymerize the easilypolymerizable compound is important for operating the purificationcolumn stably. For inhibiting this polymerization, the practice ofadding such a polymerization inhibitor as phenothiazine, hydroquinone,or methoquinone has been in vogue.

[0007] When the purification column provided with the chimney tray isused in purifying the easily polymerizable compound, however, thepolymerization of the compound occurs throughout the entire chimney trayinclusive of the vapor conduit and the umbrella in spite of the use ofthe polymerization inhibitor and the polymer brings the problem ofclogging the chimney tray and preventing it from being stably operatedfor a long time. Then, in the case of the chimney tray providedconcurrently with a vapor conduit and a liquid withdrawing tube, sincethe descending liquid comes into counterflow contact with the vapor inmotion at a relatively high speed, this vapor entrains liquid splash tothe upper part of the purification column and impairs the separatingeffect of the purification column. This impairment possibly results inincreasing the content of impurities in the target product ofpurification obtained via the top of the column.

[0008] An object of this invention, therefore, is to provide a novelliquid withdrawing tray for the purification column, a purificationcolumn utilizing the tray, and a method for purification.

[0009] Another object of this invention is to provide a tray forwithdrawing the liquid from the middle portion or middle level of thepurification column, a purification column provided with the tray, and amethod for purification such an easily polymerizable compound as(meth)acrylic acid by the use of this purification column.

[0010] Still another object of this invention is to provide, inpurification such an easily polymerizable compound as (meth)acrylicacid, a novel liquid withdrawing tray adapted to allow effectiveprevention of the polymerization and enable the purification to beperformed stably for a long time, a purification column provided withthe tray, and a method for purifying the easily polymerizable compoundby using the purification column.

SUMMARY OF THE INVENTION

[0011] The objects mentioned above are accomplished by the followingitems (1)-(10).

[0012] (1) A liquid withdrawing tray for a purification column,comprising a liquid receiving plate and a liquid withdrawing tubecommunicating with the liquid receiving plate and serving the purpose ofwithdrawing the liquid.

[0013] (2) A tray set forth in (1) above, wherein the receiving platehas the shape of a funnel.

[0014] (3) A tray set forth in (1) or (2) above, wherein the liquidreceiving plate is provided in the upper outer periphery thereof withnotches.

[0015] (4) A tray set forth in any one of (1)-(3) above, wherein theangle of the downward slope of the liquid withdrawing tube from thehorizontal direction is in the range of 0.5°-80°.

[0016] (5) A purification column provided in the column interior thereofwith a liquid withdrawing tray set forth in any one of (1)-(4) above.

[0017] (6) A purification column set forth in (5) above, wherein thelargest cross sectional area, S1, of the liquid withdrawing tray is inthe range of 10-90% of the cross sectional area, S2, of the purificationcolumn.

[0018] (7) A purification column set forth in (5) or (6) above, whereinthe liquid receiving plate is provided thereunder with at least onespray nozzle for spraying the lower side of the liquid receiving platewith a liquid.

[0019] (8) A purification column set forth in any one of (5) -(7) above,wherein the liquid receiving plate is provided thereover with at leastone liquid injecting nozzle for injecting a liquid into the liquidsurface in the liquid receiving plate.

[0020] (9) A method for purifying an easily polymerizable compound,which comprises effecting the purification by the use of a purificationcolumn set forth in any one of (5)-(8) above.

[0021] (10) A method set forth in (9) above, wherein the easilypolymerizable compound is at least one member selected from the groupconsisting of (meth)acrylic acid and esters thereof.

[0022] In the purification column, the polymerization inhibitor ispresent mainly in the liquid phase and hardly ever in the gas phase.When the vapor in the gas phase condenses in the dry part of theinterior of the purification column, therefore, the condensate readilypolymerizes because it is exposed in a state destitute of thepolymerization initiator at a relatively high temperature in thepurification column.

[0023] In the case of the chimney tray, the lower (reverse) surface ofthe umbrella covering the vapor conduit is not always kept wet with thecondensate and, therefore, is possibly suffered to permit the appearanceof a dry part. More often than not, the polymerization occurs in the drypart under the umbrella.

[0024] The study further continued has brought a discovery that the useof a funnel-shaped tray devoid of an umbrella in the place of theconventional chimney tray deprives the tray surface of the dry part andconsequently permits effective prevention of the occurrence of thepolymerization. The present invention has been perfected on the basis ofthis knowledge.

[0025] By adopting the liquid withdrawing tray of this invention andusing the purification column provided with this tray in purifying aneasily polymerizable compound such as (meth)acrylic acid, for example,it is made possible to prevent effectively the occurrence of thepolymerization in the tray part. The purification of the easilypolymerizable compound, therefore, can be stably carried out for a longtime. In this invention, the purification column effects the separationwith high efficiency because the entrainment of liquid splash to theupper part of the purification column is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a cross section illustrating one preferred embodiment ofthe liquid withdrawing tray according to this invention.

[0027]FIG. 2 is a cross section illustrating another preferredembodiment of the liquid withdrawing tray according to this invention.

[0028]FIG. 3 is a cross section illustrating still another preferredembodiment of the liquid withdrawing tray according to this invention.

[0029]FIG. 4 is a cross section illustrating yet another preferredembodiment of the liquid withdrawing tray according to this invention.

[0030]FIG. 5 is a cross section illustrating another preferredembodiment of the liquid withdrawing tray according to this invention.

[0031]FIG. 6 is a cross section illustrating still another preferredembodiment of the liquid withdrawing tray according to this invention.

[0032]FIG. 7 is a plan view of the liquid withdrawing tray shown in FIG.6.

[0033]FIG. 8 is a cross section illustrating yet another preferredembodiment of the liquid withdrawing tray according to this invention.

[0034]FIG. 9 is a cross section illustrating another preferredembodiment of the liquid withdrawing tray according to this invention.

[0035]FIG. 10 is a cross section illustrating still another preferredembodiment of the liquid withdrawing tray according to this invention.

[0036]FIG. 11 is a partial side view illustrating one preferredembodiment of the liquid receiving plate in the liquid withdrawing trayaccording to this invention, wherein the plate is provided in thelateral face part thereof with notches.

[0037]FIG. 12 is a partial cross section illustrating one preferredembodiment of the purification column of this invention provided withthe liquid withdrawing tray.

EXPLANATION OF THE PREFERRED EMBODIMENTS

[0038] Now, the preferred embodiments of this invention will beexplained below with reference to the accompanying drawings.

[0039]FIG. 1 is a cross section illustrating one preferred embodiment ofthe liquid withdrawing tray according to this invention. A liquidwithdrawing tray 11 is composed of a liquid receiving plate 12 and aliquid withdrawing tube 13 communicating with the bottom part of theliquid receiving plate and serving the purpose of withdrawing the liquidin the liquid receiving plate out of a purification column (not shown).The lateral shape (the shape of longitudinal cross section) of thisliquid receiving plate 12 does not need to be particularly discriminatedbut has only to be capable of accommodating the liquid in thepurification column (not shown) and allowing the liquid to overflow theliquid receiving plate 12 in the proximity of the upper end partthereof, wet the lower lateral face of the tray, and prevent the lowerlateral face from producing a dry part. Generally, this lateral shape ispreferred to resemble a funnel as illustrated in FIG. 1 and the planview of the liquid receiving plate 12 is a circle, for example. Theliquid withdrawing tube 13 in this case is so constructed as to extendin the perpendicular direction from the lowermost part of the liquidreceiving plate 12, begin to slope down at a point halfway along theentire length thereof, and communicate with the exterior of thepurification column (not shown).

[0040] The plan shape of the liquid receiving plate 12 has no particularrestriction either but has only to be capable of being set in thepurification column. As typical examples of the plan shape, a circle, asemicircle, an ellipsis, a square, and a broken circle may be cited.Among other shapes mentioned above, the circle is used particularlyadvantageously.

[0041] FIGS. 2-8 are schematic cross sections illustrating preferredembodiments of the liquid withdrawing tray using the same liquidreceiving plates as the funnel-shaped liquid receiving plate illustratedin FIG. 1.

[0042] Specifically, in a liquid withdrawing tray 21 illustrated in FIG.2, a liquid receiving plate 22 has a funnel-shaped longitudinal crosssection similarly in the embodiment of FIG. 1 and a liquid withdrawingtube 23 descends in an oblique direction from the bottom part of theliquid receiving plate 22.

[0043] In a liquid withdrawing tray 31 illustrated in FIG. 3, a liquidreceiving plate 32 has a cylindrical shape containing a converging partin the lower portion thereof. From the bottom part of the liquidreceiving plate 32, a liquid withdrawing tube 33 descends and turns in asubstantially horizontal direction at a point halfway along the entirelength thereof and communicates with the exterior.

[0044] In a liquid withdrawing tray 41 illustrated in FIG. 4, a liquidreceiving plate 42 is shaped like a funnel similarly in the embodimentof FIG. 1 and a liquid withdrawing tube 43 descends from the bottom partof the liquid receiving plate 42 and turns in a substantially horizontaldirection at a point halfway along the entire length thereof andcommunicates with the exterior similarly in the embodiment of FIG. 3.

[0045] In a liquid withdrawing tray 51 illustrated in FIG. 5, a liquidreceiving plate 52 is shaped like a bowl and a liquid withdrawing tube53 is similar to the liquid withdrawing tubes of FIG. 3 and FIG. 4.

[0046] A liquid withdrawing tray 61 illustrated in FIG. 6 is constructedby providing a liquid withdrawing tube 63 similar to those of FIG.3-FIG. 5 for a liquid receiving plate 62 comprising one side inclinedfrom a perpendicular plane at a point halfway along the entire lengththereof, the another side inclined from the upper through the lower endthereof (See FIG. 7).

[0047] In a liquid withdrawing tray 71 illustrated in FIG. 8, a liquidreceiving plate 72 is shaped like a pot having a narrow opening and aliquid withdrawing tube 73 is similar to those of FIG. 3-FIG. 6. Then,the liquid receiving plate 72 is provided below the lower lateral sidethereof with at least one spray nozzle 74 for spraying a liquid to thelower lateral side thereof, preferably a plurality of such spray nozzlesspaced by an equal interval, to ensure the wetting of the lower lateralside. This liquid receiving plate 72 communicates with a liquid supplyconduit 75.

[0048] In a liquid withdrawing tray 81 illustrated in FIG. 9, a liquidreceiving plate 82 is shaped like a wine glass having a narrow openingand a liquid withdrawing tube is similar to those of FIG. 3-FIG. 8.Then, the liquid receiving plate 82 is provided below the lower lateralside thereof with at least one spray nozzle 84 for spraying a liquid tothe lower lateral side thereof, preferably a plurality of such spraynozzles spaced by an equal interval, to ensure the wetting of the lowerlateral side. This liquid receiving plate 82 communicates with a liquidsupply conduit 85.

[0049] Above mentioned Figures show embodiments of funnel type liquidreceiving plate, and the funnel type means a liquid receiving platehaving a tapered portion at least one portion of the bottom. The liquidin the liquid receiving plate flows down smoothly without storage, sopolymerization of the liquid is inhibited.

[0050]FIG. 10 shows an embodiment of a dish type liquid receiving plateand the liquid receiving plate 92 has the shape of a cylinder or asquare cup and a liquid withdrawing tube 93 is similar to those of FIG.3-FIG. 9. Then, the liquid receiving plate 92 is provided below thelower lateral side thereof with at least one spray nozzle 94 forspraying a liquid to the lower lateral side thereof, preferably aplurality of such spray nozzles spaced by an equal interval, to ensurethe wetting of the lower lateral side. This liquid receiving plate 92communicates with a liquid supply conduit 95.

[0051] In the liquid receiving plates illustrated in FIG. 1-FIG. 10,those illustrated in FIG. 1-FIG. 6 are used particularly advantageouslybecause their lower lateral sides are wetted throughout with the liquidoverflowing the liquid receiving plates near the upper ends thereof andare substantially incapable of producing a dry part. They also enabletheir pertinent purification columns to effect the expected separationwith an exalted efficiency because they reduce the entrainment of liquidsplash to the upper parts of the purification columns.

[0052] The liquid receiving plates illustrated in FIG. 8-FIG. 10, forthe purpose of ensuring the wetting of the lower lateral sides thereof,may be provided with at least one spray nozzle 74 for spraying theliquid or a plurality of such spray nozzles spaced with an equalinterval. As the liquid used for spraying, it is advantageous towithdraw the liquid in the liquid receiving plate through the liquidwithdrawing tube and circulate it to the spray nozzle because the use ofthis liquid in this manner has only sparing effect to bear on thematerial balance within the purification column.

[0053] The liquid withdrawing tube is commended to be inclined downwardfrom the horizontal direction with an angle, (refer to FIG. 1), in therange of 0.5°-80°, preferably 1°-70°, and particularly preferably2°-40°, lest the liquid should remain on the surface of the liquidwithdrawing tube during a suspension of the operation of thepurification column. If this angle is less than 0.5°, the shortage willbe at a disadvantage in suffering the liquid to remain readily becauseof insufficient descent of the liquid. Conversely, if the angle exceeds80°, the excess will be at a disadvantage in the fact that a certaindistance for a height direction in the column is required until theliquid is discharged, and as a result storage time in the gas phasebecomes long, so the gas is apt to polymerize in the gas phase.

[0054] The liquid withdrawing tray according to this invention isreferred to be provided in the outer peripheral edge thereof withnotches so as to promote the overflow of the liquid from the liquidreceiving plate and keep the lateral side of the tray wet throughout thesurface. FIG. 11 is a partially longitudinal cross section of a liquidreceiving plate 102 which is provided with notches 106. Though theintervals between the notches and the widths and the largest depths ofthe notches are not generally definite because they are variable as withthe size of the liquid receiving plate, the intervals between thenotches (the distances between the bottom parts of the adjacent notches)are preferred to be in the range of 2-100 mm, preferably 5-50 mm, thewidths of the notches to be in the range of 2-100 mm, preferably 5-50mm, and the largest depths of the notches to be in the range of 2-100mm, preferably 5-50 mm.

[0055] Incidentally, the shape of such notches does not need to beparticularly limited and may be properly selected as from among atriangle (V-shape), a square, and a semicircle.

[0056]FIG. 12 is an explanatory cross section of the purification columnof this invention which is furnished with the liquid withdrawing traymentioned above. Specifically, in a shelf type purification column 130provided with a tray 131, a liquid withdrawing tray 121 provided with aliquid receiving plate 122 having a liquid withdrawing tube 123connected to the bottom part thereof is supported by a liquidwithdrawing tray-supporting plate 133.

[0057] As regards the size of the liquid withdrawing tray, the largestcross section (such as the area denoted by the reference numeral S1 inFIG. 3 and FIG. 8) is properly in the range of 10-90%, preferably20-80%, of the cross section (S2) of the purification column. That is,the quotient of S1/S2 (×100) is preferred to fall in the range mentionedabove. If the quotient exceed 90%, the excess will beat a disadvantagein increasing the pressure loss due to a decrease in the vapor flow pathand tending to induce the entrainment of liquid splash due to anincrease in the linear velocity of the vapor. Conversely, if thequotient is less than 10%, the shortage will be at a disadvantage inpreventing the purposes of such actions as mixing, cooling, heating, anddrawal of an intermediate product from being fully attained.

[0058] Part of the liquid received in the liquid withdrawing tray 121 iswithdrawn in a prescribed unit volume from the purification column andthe remainder thereof is left overflowing the tray 121 across the upperedge thereof. For the purpose of enabling the liquid to overflow thetray 121, it suffices to fix the amount of the liquid to be withdrawn tothe exterior in the range of 5-80%, preferably 20-70%, of the amount ofthe reflux liquid in the column. The overflowing liquid keeps fallinginto the tray laid beneath while wetting the lower lateral side of theliquid withdrawing tray 121. By there flux liquid containing thepolymerization inhibitor, the lateral side of the liquid withdrawingtray 121 is kept wet and protected against polymerization. This overflowis promoted by the provision of the notches mentioned above.

[0059] For the purpose of effectively preventing the polymerization, thepurification column may be provided with spray nozzles 124 adapted tospray part of the liquid withdrawn from the purification column againstthe lower lateral side of the tray 121. The nozzles communicate with aliquid supply conduit 125. Besides or simultaneously with the spraynozzles 124, the purification column may be provided with at least oneliquid injection nozzle 135 adapted to inject the liquid into the liquidsurface in the liquid withdrawing tray 121. This nozzle communicateswith a liquid supply conduit 136. By injecting the liquid through thisliquid injecting nozzle 135, it is made possible to keep the liquid inthe liquid receiving plate stirred and prevent the liquid fromstagnating and inducing polymerization. Incidentally, a spray nozzle isadvantageously used as the liquid injection nozzle 135. When a separatefeed liquid is simultaneously sprayed, the efficiency of separation isimproved because the mixing of the reflux liquid in the column with thefeed liquid is promoted.

[0060] When the liquid withdrawing tray in accordance with the presentinvention is used, in the purification column for the easilypolymerizable compound, it is useful for the following objects. In asystem where polymer generates, it is possible to discharge the liquidform the column, to separating a polymer by a solid-liquid separator andto return only the liquid into the column. Further, when a liquidcomposition between a reflux liquid in the column and a fed liquid isconsiderably different or the reflux liquid has two liquid phases,polymerization in the column can be inhibited by mixing uniformly.Furthermore, it becomes possible to inhibit the polymerization andproceed the reaction by withdrawing the liquid in the column, adding thepolymerization inhibitor and a reaction product, and returning it to thetray.

[0061] The method of this invention for the purification of an easilypolymerizable compound can be implemented in an ordinary way except thatthe use of the purification column mentioned above. In thisspecification, the “purification” means distillation (rectification),stripping and absorption, and the “purification column” meansdistillation (rectification) column, stripper and absorber.

[0062] The liquid withdrawing tray and the purification column inaccordance with this invention are used particularly advantageously inhandling an easily polymerizable compound. The easily polymerizablecompounds include (meth)acrylic acid and esters thereof. As typicalexamples of the compound, alkyl esters having alkyl groups of 1-8 carbonatoms such as methyl esters, ethyl esters, n-propyl esters, isopropylesters, n-butyl esters, n-hexyl esters, and 2-ethylhexyl esters,cycloalkyl esters such as cyclohexyl esters, hydroxyalkyl esters of 2-3carbon atoms such as 2-hydroxyethyl esters, 2-hydroxypropyl esters, and3-hydroxypropyl esters, and alkyl or dialkylaminoalkyl esters such asmethylaminoethyl esters, ethylaminoethyl esters, dimethylaminoethylesters, and diethylaminoethyl esters may be cited. These compounds maybe used for purification either singly or in the form of a mixture or ain the form of a solution containing them.

[0063] Now, this invention will be described more specifically belowwith reference to working examples.

EXAMPLE 1

[0064] A funnel-shaped liquid withdrawing tray fulfilling the followingparticulars and illustrated in FIG. 1 was installed between the 15^(th)and the 16^(th) shelf of a distillation column made of stainless steel(SUS 304) having an inside diameter of 1500 mm and containing a total of50 shelves.

[0065] Diameter of the uppermost part (outer peripheral edge): 1100 mm

[0066] Cross section of tray/cross section of column (S1/S2): 0.54

[0067] Notches in the outer peripheral edge: V notches 10 mm in width,10 mm in pitch, and 10 mm in depth

[0068] Inclination (α) of withdrawing tube: 1°

[0069] The distillation column mentioned above was kept supplied withacrylic acid containing maleic acid as impurity at a concentration of0.8% by weight at a feed rate of 9 tons per hour and was operated fircontinuous distillation under the conditions of 0.6 of reflux ratio, 35mmHg of working pressure, and 63° C. of working temperature. The liquidwithdrawn to the exterior from the funnel-shaped liquid withdrawing traywas sprayed on the lateral side of the tray and the liquid surfacerespectively at a rate of 1 ton per hour.

[0070] During the course of the distillation, phenothiazine as apolymerization inhibitor was added to the feed liquid to thedistillation column in an amount calculated to account for aconcentration of 100 ppm. A gas containing oxygen was continuouslysupplied in a prescribed amount to the interior of the distillationcolumn via the bottom thereof.

[0071] This apparatus was used to continue the distillation for 1 month.In this while, the distillate from the column top was found to containmaleic acid as an impurity at a concentration of 0.1% by weight. Whenthe interior of the column was visually inspected after 1 month'soperation, the amount of a polymer formed in the vicinity of thefunnel-shaped withdrawing tray inclusive of the liquid withdrawing tubewas found to be about 1 kg.

EXAMPLE 2

[0072] Continuous distillation was carried out by following theprocedure of Example 1 while omitting the spray of the liquid to thelateral side of the funnel-shaped liquid withdrawing tray.

[0073] When the interior of the column was visually inspected after 1month's continuous distillation, the amount of a polymer formed in thevicinity of the funnel-shaped withdrawing tray inclusive of the liquidwithdrawing tube was found to be about 1.2 kg.

EXAMPLE 3

[0074] Continuous distillation was carried out by following theprocedure of Example 1 while using a funnel-shaped liquid withdrawingtray containing no notch in the outer peripheral edge (with no otherchange).

[0075] When the interior of the column was visually inspected after 1month's continuous distillation, the amount of a polymer formed in thevicinity of the funnel-shaped withdrawing tray inclusive of the liquidwithdrawing tube was found to be about 1.2 kg.

EXAMPLE 4

[0076] Continuous distillation was carried out by following theprocedure of Example 1 while using a funnel-shaped liquid with drawingtray having no inclination in a liquid withdrawing tube (α=0) (with noother change).

[0077] When the interior of the column was visually inspected after 1month's continuous distillation, the amount of a polymer formed in thevicinity of the funnel-shaped withdrawing tray inclusive of the liquidwithdrawing tube was found to be about 1.1 kg.

[0078] Control 1

[0079] Continuous distillation was carried out by following theprocedure of Example 1 while using a chimney tray (cross section oftray/cross section of column=0.56, equivalent to a funnel-shaped liquidwithdrawing tray measuring 1100 mm in inside diameter) provided with avapor conduit 1000 mm in diameter in the place of the funnel-shapedliquid withdrawing tray.

[0080] Duringthelmonth's continued operation, the distillate emanatingfrom the column top was found to contain maleic acid as an impurity at aconcentration of 0.2% by weight. When the interior of the column wasvisually inspected after 1 month's continuous purification, the amountof a polymer formed in the vicinity of the chimney tray inclusive of theliquid withdrawing tube was found to be about 3 kg.

[0081] The results of the examples 1-4 and the control 1 arecollectively shown in Table 1. TABLE 1 Example 1 Example 2 Example 3Example 4 Control 1 Inside diameter of distillation column mm 1500 ← ← ←← Number of shelves 50 ← ← ← ← Shelves used for intermediate extraction15-16 ← ← ← ← Liquid withdrawing tray mm funnel type ← ← ← riser typeInside diameter of tray 1100 ← ← ← 1100 mm Working pressure mmHg 35 ← ←← ← Working temperature ° C. 63 ← ← ← ← Spray of lateral side of trayyes no yes ← ← Notch yes ← no yes ← Inclination of withdrawing nozzleyes ← ← no yes Maleic acid contained in feed liquid to Wt % 0.8 ← ← ← ←distillation column Polymerization inhibitor (phenothiazine) ppm 100 ← ←← ← (After completion of 1 month's continued operation) Maleic acid indistillate Wt% 0.1 ← ← ← 0.2 Polymer in column kg 1 1.2 1.2 1.1 3

[0082] Comparison of the results of the working examples and those ofthe control reveals that the use of the liquid withdrawing tray of thisinvention permitted prevention of polymerization and allowed improvementin the efficiency of separation in the distillation column.

[0083] The entire disclosure of Japanese Patent Application No.11-277199 filed on Sep. 29, 1999 including specification, claims andsummary are incorporated therein by reference in its entirely.

What is claimed is:
 1. A liquid withdrawing tray for a purificationcolumn, comprising a liquid receiving plate and a liquid withdrawingtube communicating with said liquid receiving plate and serving thepurpose of withdrawing the liquid.
 2. A tray according to claim 1,wherein said receiving plate has the shape of a funnel.
 3. A trayaccording to claim 1, wherein said liquid receiving plate is provided inthe upper outer periphery thereof with notches.
 4. A tray according toclaim 1, wherein the angle of the downward slope of said liquidwithdrawing tube from the horizontal direction is in the range of0.5-80°.
 5. A purification column provided in the column interiorthereof with a liquid withdrawing tray according to any one of claims1-4.
 6. A purification column according to claim 5, wherein the largestcross sectional area, S1, of said liquid withdrawing tray is in therange of 10-90% of the cross sectional area, S2, of said purificationcolumn.
 7. A purification column according to claim 5, wherein saidliquid receiving plate is provided thereunder with at least one spraynozzle for spraying the lower side of said liquid receiving plate with aliquid.
 8. A purification column according to claim 5, wherein saidliquid receiving plate is provided thereover with at least one liquidinjecting nozzle for injecting a liquid into the liquid surface in saidliquid receiving plate.
 9. A method for purifying an easilypolymerizable compound, which comprises effecting the purification bythe use of a purification column set forth in claim
 5. 10. A methodaccording to claim 9, wherein said easily polymerizable compound is atleast one member selected from the group consisting of (meth)acrylicacid and esters thereof.